MAKING  CONCRETE 
WATERPROOF 


vv 


By 


IRA  O.  BAKER,  M.  Am.  Soc.  C.  E. 

Consulting  Engineer 


Professor  of  Civil  Engineering 
University  of  Illinois 
Urbana,  111. 


Reprint  from  "The  Technograph”  (the  annual  of  the  Engineering 
Societies  of  the  University  of  Illinois),  No.  23,  1908-09. 


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MAKING  CONCRETE  WATERPROOF.1 
By  Ira  0.  Baker,2  M.  Am.  Soc.  C.  E. 


Concrete  is  very  largely  employed  in  many  building 
constructions,  and  in  some  situations  it  is  very  important 
that  it  should  be  at  least  practically  waterproof.  There 
are  several  methods  of  rendering  concrete  more  or  less 
impermeable  to  water,  some  of  which  are  simple  and  free 
to  any  one,  and  some  of  which  consist  in  using  secret  or 
patented  compounds.  Of  the  former  the  simplest  method 
consists  in  mixing  hydrated  lime  or  finely  divided  clay 
with  the  concrete,  thus  at  least  partially  filling  the  voids 
or  interstices  in  the  concrete  and  decreasing  the  percola- 
tion of  water.  Another  simple  method  consists  in  mixing 
powdered  alum  with  the  cement  and  dissolving  ordinary 
soap  in  the  water  to  be  used  in  mixing  the  concrete.  Al- 
though the  latter  materials  have  been  used  for  more  than 
60  years  as  a wash  for  rendering  masonry  impervious  to 
water,  and  although  in  recent  years  they  have  frequently 
been  employed  as  ingredients  of  concrete  to  make  the 
entire  mass  impermeable,  the  proportions  to  be  employed 


i 


1 From  “The  Technograph”  (the  annual  of  the  Engineering  Socie- 
ties of  the  University  of  Illinois),  No.  23,  1908-09. 

2 Professor  of  Civil  Engineering,  University  of  Illinois,  Urbana,  111. 


1 


and  the  reason  for  the  effect  seems  to  have  had  little  or  no 
scientftfc  consideration,  .at  least  the  proportions,  used  in 
practice  vary  very  widely.  For  the  purpose  of  securing 
data  for  the  use  in  the  revised  edition  of  the  writer’s 
Treatise  on  Masonry  Construction,  now  in  preparation, 
certain  investigations  and  experiments,  have  been  made ; 
and  it  is  the  object  of  this  article  to  present  these  results 
more  fully  than  can  be  done  in  the  book  mentioned. 

Alum  and  Soap  Waterproofing  Compound 
To  use  the  alum  and  soap  method  of  decreasing  the 
permeability  of  mortar  or  concrete,  the  alum  in  powdered 
form  may  be  mixed  with  the  dry  cement  or  the  sand,  and 
the  soap  may  be  dissolved  in  the  water  employed  in  mix- 
ing the  concrete;  or  both  the  alum  and  the  soap  may  be 
dissolved  in  the  water.  The  former  is  probably  the  safer 
method  in  practice,  since  with  the  latter  method  the  water 
must  be  thoroughly  stirred  while  the  two  are  being  mixed 
or  the  precipitate  may  form  in  large  masses  which  it  is 
practically  impossible  to  break  up ; and  further  the  water 
must  be  stirred  continuously  to  prevent  the  compound 
from  accumulating  on  the  surface.  These  are  conditions 
that  it  is  not  always  easy  to  be  certain  of  securing.  How- 
ever, the  alum  is  more  easily  dissolved  than  the  soap ; and 
hence  the  alum  may  be  dissolved  in,  say  one  fifth  of  the 
water  and  the  soap  in  the  remaining  four  fifths,  and  then 
the  two  portions  may  be  mixed  together,  being  careful  to 
stir  them  as  the  mixing  progresses.  The  alum  and  the  soap 
combine  and  form  a finely-divided,  flocculent,  insoluble, 


2 


water-repelling  compound  wliicli  fills  the  pores  of  the  con- 
crete and  decreases  its  permeability. 

The  best  proportions  are : alum  1 part  and  hard  soap 
2 parts,  both  by  weight.  Soap  varies  in  its  chemical  com- 
position, and  hence  a single  proportion  can  not  be  stated 
which  will  be  chemically  exact  for  all  cases.  The  above 
proportion  is  in  round  number  the  relative  combining 
weights  of  alum  and  average  hard  soap;  and  hence  it  is 
the  best  proportion  to  use,  although  widely  different  pro- 
portions have  been  used  in  practice  with  success.  Any 
reasonably  pure  soap  will  do;  but  if  soft  soap  is  em- 
ployed, a greater  amount  should  be  used  in  proportion  to 
the  amount  of  water  in  it. 

It  is  difficult  to  dissolve  more  than  about  3%  of  hard 
soap  in  cold  water;  and  hence  this  practically  limits  the 
amount  of  alum  to  1.5%  and  of  soap  3%.  These 
amounts  will  give  a precipitate  equal  to  about  3%  of  the 
weight  of  the  total  water.  The  amount  of  precipitate 
formed  in  the  pores  of  the  mortar  or  concrete  will  de- 
pend upon  the  amount  of  water  used  in  the  mixing.  Of 
course,  if  it  were  desired  to  use  a greater  quantity  of 
soap  and  alum,  the  soap  could  be  first  dissolved  in  a simi- 
lar quantity  of  hot  water,  which  is  afterwards  mixed  with 
the  water  used  in  making  the  mortar ; but  this  is  hardly 
practicable,  nor  necessary,  as  will  appear  presently. 

An  excess  of  alum  does  no  harm,  since  alum  alone  is 
a fair  waterproofing  material.  An  excess  of  soap  does 
no  harm;  and  an  excess  is  better  than  a deficiency,  since 


3 


the  excess  will  unite  with  the  free  lime  of  the  cement 
and  form  calcium  soap— a finely-divided,  water- repelling 
compound  which  is  apparently  the  essential  element  of 
several  of  the  proprietary  waterproofing  compounds.  The 
above  is  the  explanation  why  such  diverse  proportions 
of  alum  and  soap  give  reasonably  successful  results  in 
actual  practice. 

Effect  of  Water-Repelling  Compound. 

A film  of  oil  on  the  wires  of  a moderately  fine  sieve 
makes  it  nearly,  if  not  quite,  water-tight.  The  question 
then  naturally  occurs : Can  a water- repelling  compound 
in  the  concrete  act  in  the  same  way  as  oil  on  a sieve? 
Or  in  other  words,  can  a volume  of  water-repelling  com- 
pound less  than  the  volume  of  the  voids  in  the  concrete 
decrease  the  permeability  of  the  concrete  in  a greater 
ratio  than  the  per  cent  of  the  voids  filled?  If  so,  then 
a water-repelling  compound  is  more  efficient  in  decreas- 
ing the  permeability  of  a concrete  than  mere  void-filling 
material. 

To  test  the  effect  of  a water-repelling  ingredient  upon 
the  permeability  of  concrete,  a series  of  experiments  were 
made  under  the  writer’s  direction  by  Mr.  B.  L.  Bowling 
in  the  Cement  Laboratory  of  the  University  of  Illinois. 
The  experiments  consisted  in  molding  a series  of  port- 
land  cement  mortar  disks  in  short  lengths  of  6-in.  pipe, 
subjecting  these  disks  to  water  under  pressure,  and 
measuring  the  amount  of  percolation.  For  convenience 
in  making  the  experiments,  mortar  was  used  instead  of 


4 


concrete.  The  mortar  for  the  treated  and  the  untreated 
disks  was  alike  except  for  the  alum  and  soap  compound. 
However,  the  mortars  containing  the  alum  and  the  soap 
were  invariably  drier  than  those  made  without  these  in- 
gredients; but  this  probably  has  no  significance  in  this 
connection.  An  annulus,  1-in.  wide,  at  the  circumference 
of  both  the  top  and  the  bottom  faces  of  the  disk  was 
coated  with  hot  asphalt,  and  also  covered  with  a rubber 
gasket  against  which  a flat  casting  was  pressed  by  holts 
through  an  external  flange;  and  consequently  the  flow 
was  through  a disk  4 ins.  in  diameter  and  2 ins.  thick. 
The  percolating  water  was  caught  in  a tin  funnel,  the  top 
of  which  fitted  closely  against  the  lower  casting  and  the 
neck  of  which  passed  through  a perforated  rubber  stop- 
per into  a bottle.  The  water  pressure  varied  from  40 
to  45  lb.  per  sq.  in.  The  data  and  the  results  of  the 
experiment  are  given  in  the  table  on  page  7.  Tests 
1 and  2 were  made  together,  as  also  tests  3 to  6,  and  7 
to  9.  The  intention  was  to  have  four  tests  in  each  series, 
but  through  one  reason  or  another  some  of  the  tests  were 
of  doubtful  value  or  were  useless,  and  hence  are  not 
reported. 

The  mortar  used  in  making  the  disks  was  1 : 6,  which 
is  unusually  lean  and  porous,  but  it  was  purposely  made 
porous  the  better  to  test  the  effect  of  the  alum  and  soap 
compound.  Known  weights  of  cement,  sand,  and  water 
were  used,  and  the  volume  of  the  mortar  produced  was 
measured;  and  then  knowing  the  specific  gravity  of  the 


5 


several  ingredients,  the  density  of  the  resulting  mortar 
was  computed  by  a process  not  necessary  to  explain 
here.  The  voids  in  each  disk  are  stated  in  the  table,  and 
ranged  between  23.4  and  24.0%.  If  the  sand  had  not 
been  well  graded,  the  per  cent,  of  voids  would  have  been 
considerably  greater,  and  also  the  individual  voids  would 
have  been  larger.  A 1 : 2 : 4 concrete  made  of  well-graded 
sand  and  stone  would  have  only  about  13  to  15  % of  voids. 

The  amount  of  water  used  in  making  each  of  the  sev- 
eral disks  is  stated  in  the  table.  Knowing  the  percentage 
of  alum  and  soap  in  the  water  and  the  amount  of  water 
used  in  mixing,  it  is  easy  to  compute  the  amount  of  the 
precipitate  in  the  mortar.  The  amount  of  precipitate  in 
each  disk  is  given  in  the  table. 

The  percolation  for  each  of  several  successive  24-hour 
periods  is  given  in  the  table.  The  results  show  that  the 
alum  and  soap  precipitate  is  quite  effective  in  decreas- 
ing the  percolation.  In  the  first  series  of  experiments., 
No.  1 and  2,  the  alum  and  soap  compound  equal  to  1.2% 
of  the  cement,  stopped  a little  over  76%  of  the  percola- 
tion during  the  first  24  hours ; and  the  corresponding  re- 
sults for  the  other  series  are  54  and  69%,  respectively. 
The  variation  in  the  several  series  is  surprisingly  large, 
but  it  is  certain  that  the  experiments  were  carefully 
made;  and  the  range  in  the  results  probably  indicates 
the  effect  of  undetected  inequalities  in  the  materials,  the 
proportions,  and  the  mixing. 


6 


TABLE  SHOWING  EFFECT  OF  ALUM  AND  SOAP  ON  PERMEABILITY  OF  CEMENT  MORTAR. 


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Somewhat  similar  results  were  obtained  for  each  of 
the  successive  24-hour  periods.  The  decrease  in  percola- 
tion through  the  treated  and  also  through  the  untreated 
disks,  is  due  to  two  causes,  viz : 1.  The  percolating  water 
doubtless  has  in  it  some  solid  matter,  which  clogs  the 
pores  of  the  upper  surface  of  the  disk.  However,  the 
water  employed  was  that  in  the  city  water  mains  and 
was  reasonably  free  from  solids.  2.  The  percolating 
water  takes  up  the  soluble  portions  of  the  mortar  from 
the  upper  part  of  the  disk  and  carries  them  to  the  lower 
part.  It  is  universal  experience  that  all  masonry  be- 
comes more  nearly  water-tight  after  use.  It  is  interest- 
ing to  note  that  in  this  series  of  experiments  the  aver- 
age percolation  during  the  twelfth  24-hour  period  was 
only  2.6%  of  that  during  the  first  period. 

The  most  interesting  feature  of  these  experiments  is 
that  the  alum  and  soap  compound  equal  to  an  average 
of  1.2%  of  the  cement  in  a mortar  containing  an  average 
of  23.7%  voids,  stopped  65%  of  the  percolation;  or,  in 
other  words,  adding  water-repelling  void-filling  material 
equal  to  approximately  5%  of  the  voids  reduced  the  per- 
colation to  one  third  of  that  of  the  untreated  mortar.  One 
can  hardly  conceive  that  an  equal  amount  of  a non-re- 
pelling void-filling  material  would  be  equally  effective. 
Apparently  then  the  alum  and  soap  compound  in  the 
concrete  acts  like  oil  on  the  wires  of  a sieve.  The  ex- 
periments are  being  continued  to  determine  the  effect 
of  the  alum  and  soap  compound  with  less  porous  mortars. 


8 


The  mixing  of  alum  and  soap  in  the  concrete  reduces 
its  strength  somewhat;  hut  there  are  many  situations  in 
which  strength  is  unimportant,  or  at  least  is  less  im- 
portant than  water-tightness.  The  effect  of  the  alum  and 
soap  upon  the  strength  of  the  mortar  varies  a little  with 
the  method  of  storing  the  test  samples.  For  example, 
the  mean  of  six  neat  portland  cement  briquettes  mixed 
with  21%  of  water  which  contained  %%  of  alum  and 
1 y2%  of  new  “Ivory”  soap,  when  left  in  the  molds  1 day 
and  stored  in  a moist  chamber  of  6 days,  had  a strength 
of  87%  of  that  of  similar  briquettes  made  with  water 
alone ; and  when  left  in  the  molds  one  day  and  then  stored 
6 days  in  water  had  84% ; and  when  twice  as  much  alum 
\ and  soap  were  used,  the  strength  was  83  and  71%  re- 
spectively. 


1 


) 


9 


